1. Field of the Invention
The present invention relates to novel tin curing catalysts for converting organopolysiloxane compositions into silicone elastomers, and, more especially, to such catalysts comprised of a mixture of a diorganotin bis(.beta.-diketonate) and an organotin (IV) compound devoid of any .beta.-diketonato functional group.
2. Description of the Prior Art
A wide variety of tin compounds have already been proposed to this art as catalysts for crosslinking polyorganosiloxane compositions and in particular RTV compositions (compositions which can be vulcanized at ambient temperature) in a single pack or in two packs, otherwise known as single- or two-component compositions.
The most commonly employed compounds are tin carboxylates, such as tributyltin monooleate, tin 2-ethylhexanoate or dialkyltin dicarboxylates, such as dibutyltin dilaurate and dibutyltin diacetate (see the Noll text Chemistry And Technology Of Silicones, page 337, 2nd Edition, Academic Press (1968).
In U.S. Pat. No. 3,186,963, the tin catalyst proposed for such purpose is the reaction product of a tin salt, especially dibutyltin dilaurate, with ethyl polysilicate.
According to U.S. Pat. No. 3,862,919, the tin catalyst proposed is the reaction product of a dialkyldialkoxysilane with a tin carboxylate.
According to Belgian Pat. No. 842,305, the catalyst thus proposed is the reaction product of an alkyl silicate or of an alkyl trialkoxysilane with dibutyltin diacetate.
In U.S. Pat. No. 3,708,467 a catalytic system is described consisting of a mixture of certain tin salts with a specific titanium chelate, in a single-component composition.
Lastly, in published European patent application EP-A-147,323 and in U.S. Pat. Nos. 4,517,337 and 4,554,310 there is described the use of diorganotin bis(.beta.-diketonates) for crosslinking neutral single-component compositions (U.S. Pat. No. 4,517,337 and U.S. Pat. No. 4,554,310), or for single- and two-component compositions (EP-A-147,323).
Although EP-A-147,323 has enabled considerable progress in the search for a tin catalyst capable of being used both for single- and two-component compositions, it has become apparent that diorganotin bis(.beta.-diketonate) compounds exhibit a core setting time which is somewhat slow, especially for two-component compositions.
In general, in the case of the single-component compositions, a basic problem which is encountered is that of the storage stability and of the retention of the physicochemical properties (extrudability, castability, setting time) of the composition, and of retention of these same properties by the crosslinked material (mechanical properties, hardness, elongation, tear strength, adhesiveness, and the like).
Serious need exists in this art for a catalyst which crosslinks very rapidly in moist air and at the surface, but which at the same time ensures a core crosslinking which is as thorough as possible, and which is active in low concentrations, while minimizing the decomposition reactions of the crosslinked material, which are inherent in the presence of tin.
Insofar as the product crosslinked material is concerned, the same problems exist in the case of the single-component compositions, but, in addition, the process time, namely, the time during which the composition can be used after mixing without hardening, must be sufficiently long to permit its use, but sufficiently short to produce a molded shaped article which can be handled not later than 24 hours after its manufacture.
Such catalyst must, therefore, enable a satisfactory compromise between the utilization time of the catalyzed mixture and the time after which the molded article can be handled. In addition, the catalyst must provide the catalyzed mixture with a spreading time which does not vary as a function of storage time.
Furthermore, the tensile properties of the crosslinked materials, especially hardness and tear strength, must remain stable under storage.